1) Field of the Invention
The present invention relates to a light-amount adjusting apparatus used in an optical system of an imaging apparatus such as a video camera.
2) Description of the Related Art
As a light-amount adjusting apparatus according to a conventional technology, to be used in an optical system in an imaging apparatus such as a video camera, there is a device where in adjusting light amount, in consideration of reduction of resolution deterioration owing to light diffraction, an ND filter and an aperture diaphragm are operated (see, for example, Japan Patent Application Publication No.2002-14384). In this device, as the illuminance of an object changes to low illuminance, middle illuminance, and high illuminance, the ND filter and the aperture diaphragm are driven independently, and light-amount adjustment is carried out. Further, there is a device where an ND filter is attached to two aperture diaphragms, and the ND filter is structured so that light transmittance should become lower from the center of light axis toward the outside in radial direction (see, for example, Japan Patent Application Publication No.8-43878). Furthermore, there is a device where an aperture opening is formed by aperture diaphragms moving on a plane perpendicular to light axis in the direction in which the aperture diaphragms become away from each other, and an ND filter having at least 2 stages of light transmittance is attached to an aperture diaphragm (see, for example, Japan Patent Application Publication No.2001-272709).
However, in the devices disclosed in the above literatures, part of the ND filter is left in the light path even at the maximum aperture (maximum opening) of the aperture diaphragms, accordingly even at the maximum aperture, light going through the opening is subject to influence of the ND filter, and sufficient light amount cannot be obtained.
Further, in the device disclosed in Japan Patent Application Publication No.2001-272709, the opening and closing mechanism of aperture diaphragms is structured so as to move in the same direction (vertical direction) as the moving frame mechanism of the ND filter. Further, the aperture and the ND filter are driven in two stages in vertical direction, accordingly the device is long in vertical direction and the size thereof is large. Furthermore, the opening and closing mechanism of aperture diaphragms and the moving frame mechanism of ND filter are driven by different driving motors, and the respective driving motors are arranged in the front and rear of the device, accordingly the device is large in the anteroposterior direction.